1. Field of the Invention
The present invention relates to a developing apparatus used for image forming apparatus of an electrophotographic or electrostatic recording type such as photocopier and a printer.
2. Related Background Art
In an image forming apparatus of electrophotographic or electrostatic recording type, developing is generally realized by causing an image bearing body bearing an electrostatic latent image to correspond to a developing sleeve (a developer carrying body) carrying developer of a developing device, and in order to generate a developing electric field between an image bearing body and a developing sleeve, a developing bias is applied to the developing sleeve.
As this developing bias, as shown in FIG. 15, the one in which an AC component is superimposed on a DC component has been used, and rectangular waves of the AC component shown in FIG. 15 with a frequency of about 2 kHz (xc2xd cycle=250 xcexcs) and a peak-to-peak voltage (Vpp) of about 2 kV are conventionally used to obtain a good developed image.
However, a developing bias as shown in FIG. 15 has disadvantages that a highlighted portion of an image xe2x80x9csuffers from coarse imagexe2x80x9d and that a sufficient image density cannot be obtained in case of a smaller particle size of toner of developer.
To remedy these disadvantages, as shown in FIG. 16, it has been proposed by the present applicant that a developing bias having an AC component intermittently superimposed on a DC component is used. However, this developing bias suffers from problems as follows.
A developing bias (to be referred to as blank pulse bias or BP bias herein) as shown in FIG. 16 presents developing performance better than a developing bias (to be referred to as a rectangular bias herein) as shown in FIG. 15, but as a condition to prevent a coarse image in a highlighted portion, it is essential that a frequency of a pulse portion is raised to not less than 4 kHz (xc2xd cycle=125 seconds). In addition, as a condition to increase image density, it is necessary that Vpp of a pulse portion is not less than 2 kV as in a conventional rectangular wave.
In case of Vpp of a pulse portion exceeding 2 kV with a frequency not less than 4 kHz, when a conductive foreign material contaminates a developing section, problems occur that an abnormal discharge takes place in that portion, ring-form spots are formed on an image, and image quality is seriously deteriorated.
Accordingly, in the case where a BP bias is used, Vpp is required to be controlled to be lower than in case of a conventional rectangular bias.
It is an object of the present invention to provide a developing apparatus that can perform sufficient development with a Vpp lower than a conventional rectangular bias.
It is another object of the present invention to provide a developing apparatus that performs good development with an appropriate developing bias, prevents a coarse image in a highlighted section, makes a sufficient image density available, and never suffers from image defects due to an abnormal discharge.
It is still another object of the present invention is to provide a developing apparatus comprising:
a developer carrying body provided to face an image bearing body, wherein the developer carrying body carries and conveys a developer having toner and carrier to a developing region; and
bias applying means for applying a developing bias to the developer carrying body, wherein the bias applying means applies to the developer carrying body the developing bias having periodically an alternate voltage portion having alternately a first voltage to generate an electric field to direct toner from the developer carrying body to the image bearing body and a second voltage to generate an electric field to direct the toner from the image bearing body to the developer carrying body and a pause portion to pause applying the alternate voltage, wherein T1 and T2 have a relationship of T1 greater than T2, where T1 is an applying time period of the first voltage and T2 is an applying time period of the second voltage.
It is yet another object of the present invention is to provide a developing apparatus comprising:
a developer carrying body provided to face an image bearing body, wherein the developer carrying body carries and conveys a developer having toner and carrier to a developing region;
bias applying means for applying a developing bias to the developer carrying body, wherein the bias applying means applies to the developer carrying body the developing bias having periodically an alternate voltage portion having alternately a first voltage to form an electric field to direct toner from the developer carrying body to the image bearing body and a second voltage to form an electric field to direct the toner from the image bearing body to the developer carrying body and a pause portion to pause applying the alternate voltage, wherein a potential difference Vg between an effective center voltage of the developing bias voltage and the first voltage is larger than a potential difference Vb between an effective center voltage of the developing bias voltage and the second voltage.
Objects of the present invention other than those described above as well as characteristics of the present invention will become more apparent by reading the following detailed description with reference to attached drawings.